Heating and cooling of electrons in an ultracold neutral plasma using Rydberg atoms

TL;DR

This study experimentally demonstrates how embedding Rydberg atoms into an ultracold neutral plasma can either heat or cool the electrons, depending on the Rydberg binding energy relative to the initial electron temperature, aligning with numerical models.

Contribution

It provides the first experimental measurement of the crossover condition between heating and cooling in ultracold plasmas with embedded Rydberg atoms, supported by a simple intuitive model.

Findings

Crossover condition at |E_b| ≈ 2.7 × k_B T_{e,i}

Agreement between experimental results and numerical modeling

Development of an intuitive picture supporting the 'bottleneck' concept

Abstract

We have experimentally demonstrated both heating and cooling of electrons in an ultracold neutral plasma (UNP) by embedding Rydberg atoms into the plasma soon after its creation. We have determined the relationship between the initial electron temperature, $T_{e,i}$, and the binding energy of the added Rydberg atoms, $E_b$, at the crossover between heating and cooling behaviors (that is, the binding energy of the atoms, which, when they are added to the plasma, neither accelerate or slow down the plasma expansion). Specifically, this condition is $|E_b| \approx 2.7 \times k_B T_{e,i}$ when the diagnostic used is the effect of the Rydberg atoms on the plasma asymptotic expansion velocity. Additionally, we have obtained experimental estimates for the amount of heating or cooling which occurs when the Rydberg binding energy does not satisfy the crossover condition. The experimental results for the crossover condition, and the degree of heating or cooling away from the crossover, are in agreement with predictions obtained from numerical modeling of the interactions between Rydberg atoms and the plasma. We have also developed a simple intuitive picture of how the Rydberg atoms affect the plasma which supports the concept of a `bottleneck' in the Rydberg state distribution of atoms in equilibrium with a co-existing plasma.